The present invention relates to an electrochromic (referred to as an EC hereafter) display device in which a display image is changed between electrodes coated with an electrochemical coloration material.
A display method of a conventional EC display device will be described in conjunction with FIG. 1.
A transparent electrode 3 is formed on an upper transparent substrate 1. A tungsten trioxide (referred to as WO.sub.3 hereafter) film 5 is formed on the transparent electrode 3. Electric charges are injected into the WO.sub.3 film by applying a negative electric potential to the transparent electrode 3 and a positive electric potential to a counter electrode 9 via an electrolyte 7 and a white background plate 8, whereby the WO.sub.3 film colors blue. The electrolyte 7 is a solution of lithium perchlorate (LiClO.sub.4) and propylene carbonate, and the white background plate 8 comprises a multipolar ceramic plate. The counter electrode 9 is made of an inert material such as gold, platinum or sintered carbon. A counter substrate 2 defines a container for sealing the electrolyte 7 and the background plate 8 with a sealing agent 10. Reference numeral 11 denotes a mask plate of the display device. The following is the formula of reaction in case the WO.sub.3 film is colored. ##STR1##
When a positive electric potential is applied to the transparent electrode and a negative electric potential is applied to the counter electrode, the electric charges inside the WO.sub.3 film are extracted, and the WO.sub.3 film is bleached as indicated by the above formula.
Thus the conventional EC display device displays images by applying a voltage between the transparent electrode on which the WO.sub.3 film is formed and the counter electrode to color or bleach the WO.sub.3 film.
This method, however, is disadvantageous in the following respects:
1. The display device is thickened by use of a particular insert material.
2. The cost is extremely high because the counter electrode is made of expensive materials such as gold, platinum and sintered carbon.
3. The panel structure is complicated and the reliability is low since the terminals of the counter electrode must be extracted to the outside of the panel structure.
Accordingly, the present invention aims to eliminate the above noted drawbacks, and therefore it is an object of the invention to facilitate the practical use of EC display devices.
An EC display device according to the present invention eliminates a counter electrode which, in the prior art, serves for coloring the EC material, and instead, applies a voltage between groups of bleached and colored display electrodes for displaying images.